This invention is particularly concerned with thermoforming operations in which a web or sheet of thermoplastic material is first softened by heat and then formed or shaped while in this stretchable plastic state by being drawn or pressed against the contours of a mold or die under the influence of a fluid pressure differential. The former may be exemplified by the application of pressure within a pressure box produced by compressed air or the like, to press the softened material against the contours of a male mold or core pin. Alternately, a vacuum may be applied from within a female mold while allowing atmospheric pressure to shape the heat softened thermoplastic material against the surface of the female mold. For many purposes, a combination of these two techniques is preferred with the softened sheet material being subjected to vacuum on the face as such combination provides greater versatility and better control in the forming operation.
Recently, a process has been advanced utilizing a preformed liner which produces a barrier container, i.e., a semi-rigid plastic container provided with a liner or inner layer of another material or materials having properties different than the outer layer thereof. In accordance with such process, the preformed liner or sleeve (manufactured, for example, by thermoforming techniques) is positioned over a core rod with a parison being subsequently formed about the liner in an injection station of an injection blow molding machine. The lined or composite parison is expanded in a blow molding station of the machine with the resulting lined container being removed from the core rod in a product receiving station. As a result of the creasing tendency of the liner during the formation of the preform due to orientation and/or size variations of the thermoformed liner, many barrier containers are formed which have poorly fitted liners.